Gas insulating devices are filled with an insulating gas, such as an SF6 gas. With conventional moisture concentration detecting devices, the moisture sensors, which detect moisture, are arranged in the gas insulating devices. The moisture sensors are configured to include porous electrodes provided to face each other and a hydrogen-ion conductive solid electrolyte membrane that is provided between the porous electrodes and is in equilibrium with the moisture concentration in an SF6 gas. The moisture concentration detecting devices measure the moisture concentration in the SF6 gas by applying an AC voltage across the porous electrodes and measuring the AC impedance between the electrodes, which changes in accordance with the moisture concentration in the SF6 gas (see Patent Literature 1).